Some electronic systems such as base transceiver stations (“BTS”) are designed using a Doherty type linear amplifier configuration for increased linearity and efficiency in a linear amplifier topology. In general, the Doherty amplifier uses at least two amplifier paths, a carrier amplifier path and a peaking amplifier path, and combines the output of these two paths at a summing junction (or node) in such a way as to provide for the increased linearity and efficiency. In the traditional Doherty amplifier topology, a carrier amplifier receives an undelayed input signal for amplification, and a peaking amplifier receives a phase delayed input signal for amplification. More recently, electronic systems such as BTSs and mobile devices are designed using a so called “inverted” Doherty amplifier configuration. In this Doherty type configuration, the carrier amplifier receives the delayed input signal for amplification, and the peaking amplifier receives the undelayed input signal for amplification.
One parameter of a Doherty type amplifier that can affect linearity and operational efficiency of the amplifier is peaking amplifier off-state impedance. More particularly, the peaking amplifier off-state impedance is the impedance looking back into the peaking amplifier, including the output match, while the peaking amplifier is in an OFF state. Peaking amplifier off-state impedance should be relatively high so as not to load the carrier amplifier during a low RFIN drive condition when the peaking amplifier is off. Known Doherty type amplifier topologies maintain a high enough peaking amplifier off-state impedance for some applications, but not for others. For example, due to a lower than desired peaking amplifier off-state impedance, efficiency and linearity are compromised for applications utilizing a Doherty amplifier configuration that includes laterally diffused metal oxide field effect transistors as the carrier and peaking amplifiers and that operate at higher frequencies (e.g., around 2 GHz).
Thus, there exists a need for a Doherty type linear amplifier configuration that addresses at least some of the shortcomings of some known Doherty type amplifier configurations.
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